A typical liquid crystal cell of a flat panel display device such as a liquid crystal display device consists essentially of a thin film transistor substrate for driving the display device, a color filter for displaying colors, and a liquid crystal therebetween. The color filter is a substrate in which a pattern is formed using a photosensitive organic material containing a pigment dispersion by photolithography and pixels are formed using three or more kinds of color inks having different transmission-absorption wavelengths. To create sharp and natural images in the liquid crystal display device, the color filter is required to have a high contrast and a minimum color difference.
The use of a black matrix is necessary to divide the pixels in the production of the color filter. However, the largest problem encountered in the formation of the black matrix pattern and the pixels is that a stepped portion is formed at the boundaries between the pixels, as illustrated in FIG. 2. The stepped portion gives rise to a difference in distance between the color filter and the thin film transistor in the liquid crystal cell, and as a result, an electric field applied is distorted, which causes a decrease in contrast and an increase in color difference.
To overcome such disadvantages, additional processing is needed. For example, an optical compensation film is formed for the purpose of preventing a decrease in contrast. Further, upon occurrence of a color difference, an additional step can be taken to compensate circuit signals. However, these additional processing steps are responsible for high production cost of the liquid crystal display device.
As an attempt to reduce the height of the stepped portion, a transparent organic thin film 100 is formed over the entire surface of the color filter, as illustrated in FIG. 3. However, the surface area of the color filter covered with the thin film is so large that impurities tend to adhere to the thin film, leading to an increase in the number of defects. Accordingly, an extremely high degree of cleanliness is needed to exclude impurities from the color filter. Further, the intended effect of the thin film is not as satisfactory as expected.
Thus, there is an urgent need to provide a color filter substrate that achieves a high contrast and a minimum color difference without the need for additional processing and extreme production conditions.